The above two applications and the present invention are directed to the problems of profoundly deaf individuals; i.e., people whose hearing loss is greater than 90 db. For most, no surgery or other treatment exists that can restore their hearing. These inventions, and especially the present invention, are directed to an auditory prosthesis which electrically stimulates neural tissue producing the sensation of sound and restoring hearing to the profoundly deaf.
An auditory prosthesis system consists essentially of four parts; the speech processing hardware, the power and data transmitter, the implanted receiver stimulator, and the implanted electrodes. The external electronics to perform the speech processing are the subject of a separate body of technology and are not part of this application. The coupling of the skin is achieved using the incorporated application entitled "A Wide Band . . . . " This system takes the digitally encoded signals and transmits them along with power through the skin and into the implanted stimulator. The stimulator, disclosed in the incorporated "Multi-Channel Implantable . . . " decodes the bidstream and delivers charge or current wave forms into electrodes implanted in the auditory nerve or cochlea.
The cochlea itself and the cochlear prosthesis are in the shape of a snail, with the electrodes which are to transmit sound to the auditory nerve spaced along the prosthesis. In fact, the prosthesis lies in the lower chamber of the cochlea called the scala tympany.
The nerves are located in this chamber, which is a liquid filled chamber, and the nerves are separated from this chamber by a bony wall. The objective is to stimulate these nerves with the current from the electrodes.
The cochlea is laid out in what is called a tonotopic fashion, i.e., the nerves at one end bringing up high frequencies and at the opposite end picking up the lower frequencies. Therefore, the objective of the cochlear prosthesis is to extend it and its electrodes along where the nerves are tonotopically organized, and stimulate the appropriate nerve to represent sounds at different frequencies. It would be hoped that by energizing a particular electrode, the adjacent nerves are stimulated to provide an inherent frequency sorting. However, in reality this ideal situation does not actually prevail due to a phenomenon called current spreading, whereby activation of any electrode will in fact stimulate many of the electrodes at a distance from the activated electrode. Not all the nerves are stimulated to the same degree, but there is not a great deal of difference.
Therefore, it is an objective of the present invention to provide an improved means of stimulating the auditory nerves in an auditory prosthesis system.
More particularly, it is an objective of the present invention to provide an improved system and method for auditory stimulation which overcomes the current spreading problem.